The present invention is related to Japanese patent application No. Hei. 11-300209, filed Oct. 21, 1999; the contents of which are incorporated herein by reference.
The present invention is related to a manufacturing method, and more particularly, to a manufacturing method for manufacturing metallic shells for spark plugs used in internal combustion engines such as those in automobiles.
A conventional method for manufacturing metallic shells for spark plugs is shown in FIGS. 9A-9G. Here, material processed, J1 through J6, shown in FIG. 9A through FIG. 9F, are formed by a cold forging operation using a molding die and a punch, for example as described in Japanese Laid Open Patent No. Hei-7 16693, 1995.
In other words, a stepped columnar material J2 is formed from a columnar material J1 by a cold forging operation. Next, material J3 having a large diameter head part J8 forming a large diameter hole, and a small diameter foot part J9 forming a smaller diameter hole is formed. The large diameter head part J8 and small diameter foot part J9 are stretched to successively form parts J4, and J5. Then, the interior of the material is bored to form part J6.
The step portion J10 located between the large diameter head part and the small diameter foot part in the material J6 is processed by a cutting operation to produce the tapered part J11 as shown in FIG. 9G. The threaded part J12 is produced by a rolling operation and the final finished product of a metallic shell J7 is produced. The metallic shell J7 is attached to the engine head by means of the threaded part J12. The tapered part J11 is tightly attached to the engine head to provide a seal between the spark plug and the engine head.
The tapered part produced by this type of cutting process, however, is a result of secondary process applied to a cold forged product. Consequently, it is necessary to make frequent checks on the processes and frequent exchange of cutting tools, to consistently obtain the exact shape. The requirements for the angle of the tapered part, deflection of the axis, and surface roughness are being maintained in this manner. Therefore, as the cutting tool approaches the end of its service life, accuracy of the finished product suffers. Additionally, there have been strong demands in recent years for lower costs, easier way to manage the tapered part, and improved consistency of the shape of the finished products in the manufacturing of metallic shells. The present invention was developed in light of these and other drawbacks.
The present invention provides a manufacturing method of producing a tubular metallic shell for a spark plug having a tapered outer periphery to provide a seal. The desired shape of the tapered part is produced by cold forging. A tubular metallic shell of a spark plug has a stepped tapered part in its outer periphery, between a larger diameter part and a small diameter part. This is to provide a seal when tightly attached to the engine head. The tapered part is formed by cold forging. The cold forging operation is performed by the processing steps include the following.
(a) In the first processing step, a first molding die having a stepped inner cavity that forms a tapered bearing surface between the lager diameter part and the small diameter part, is prepared. As a columnar material is secured in the stepped inner cavity of the first molding die, a first punch is pressed against the material in the axial direction to transform its shape. Consequently, a first processed part having a stepped columnar shape is formed. It comprises a large diameter head part with a large diameter hole opened at one end, and a small diameter foot part positioned at the other end. The small diameter foot part has a smaller outer diameter than the large diameter head part. In addition, a first tapered part located at the boundary between said large diameter head part and said small diameter foot part is formed.
(b) In the second processing step, a second molding die having a stepped inner cavity that forms a tapered bearing surface at the boundary between the large diameter part and the small diameter part is prepared. The tapered bearing surface has a greater tapering angle B than the tapering angle A in the first tapered part. A second punch having a larger outer diameter than the outer diameter of the small diameter foot part of the first processed part, described above, is also prepared. As the first processed part, mentioned above, is secured in the stepped inner cavity of the second molding die, the second punch is inserted into the larger diameter hole in the first processed part, and pressed in the axial direction. Consequently, the shape of said first tapered part is transformed to conform to the bearing surface of the second die. Thus, a second processed part having a stepped columnar shape, and a second tapered part with the tapering angle B, described above, is formed.
(c) In the third processing step, a third molding die having a stepped inner cavity that forms a tapered bearing surface at the boundary between the large diameter part and the small diameter part is prepared. The tapered bearing surface has a smaller tapering angle C than the tapering angle B in the second tapered part. A third punch with a tip having a smaller outer diameter than that of the small diameter foot part of the second processed part, described above, is also prepared. As the second processed part, mentioned above, is secured in the stepped inner cavity of the third molding die, the third punch is inserted into the large diameter hole in the second processed part, and pressed in the axial direction. Consequently, the shape of said second tapered part is transformed to conform to the bearing surface of the third molding die. Thus, a third processed part having a stepped columnar shape and a third tapered part with the tapering angle C, described above, is formed.
The tapering angles A, B, and C, refers to the angles formed between the axial direction of each processed part or stepped inner cavity, and the inclination angles of each processed material""s outer surface or each inner cavity""s inner surface. The axial direction in each processed part and each stepped inner cavity is defined as 0xc2x0. This is illustrated in FIG. 5 and FIG. 6, explained later.
In the third processing step, the shape of the second tapered part is transformed to conform to the bearing surface (thereafter called the third bearing surface) of the stepped inner cavity of the third molding die. The third bearing surface has a tapering angle C that is smaller than the tapering angle B of the second tapered part.
In the second processing step, the second punch having a larger outer diameter than the outer diameter of the small diameter foot part of the first processed part is used, as shown in FIG. 5. Therefore, the pressure exerted onto the first processed part is directly conveyed to the first tapered part. However, in the third processing step, a third punch, having a tip with an outer diameter smaller than the outer diameter of the small diameter foot part of the second processed part, is used. This is illustrated in FIG. 6. Therefore, the pressure exerted onto the second processed part is conveyed directly to the small diameter foot part of the second processed part, but not directly to the second tapered part.
In this process, the second tapered part is stretched by the transformation of the small diameter foot part, and the third tapered part is formed as a final tapered part. In the third processing step, the configuration of pressure applied at the tapered part is different from that of the second processing step. Therefore, although the tapering angle C is smaller than the tapering angle B, the lubricating oil is less likely to be retained.
In another aspect, the third molding die is pushed in a direction opposite from the direction of pressure applied by the third punch, in the third processing step. The third tapered part and the third bearing surface are forced to remain in contact even after the tapered part is formed by this arrangement. Preferably, the tapering angle B is greater than the tapering angle A by 1xc2x0 to 10xc2x0. This is because when the difference in the two tapering angles is less than 1xc2x0, escape of the lubricating oil is blocked. When the difference in the two tapering angles is more than 10xc2x0, the amount of lubricating oil retained between the first tapered part and the second bearing surface becomes so great that it is difficult to discharge it properly. Also, preferably, the tapering angle C smaller is than the tapering angle B by 0.5xc2x0 to 5xc2x0, as described in Claim 4 of the Patent Claims. Here, when the difference between the two tapering angles is less than 0.50xc2x0, the second tapered part undergoing transformation may become severed by the pressure of the third punch. When the difference between the tapering angles B and C is greater than 5xc2x0, the amount of lubricating oil retained between the first tapered part and the second bearing surface becomes so much, it is difficult to discharge it properly.
Further areas of applicability of the present invention will become apparent from the detailed description provided hereinafter. It should be understood that the detailed description and specific examples, while indicating preferred embodiments of the invention, are intended for purposes of illustration only, since various changes and modifications within the spirit and scope of the invention will become apparent to those skilled in the art from this detailed description.